1. Field of the Invention
This invention relates to a horizontal type of traveling-wave antenna with the construction of a line- or strip-conductor or a horizontal array above a ground.
2. Description of the Related Art
There are two types of horizontal antennas such as the microstrip and the wave (Beverage) in antenna similar in construction to the present invention. For ground return, the microstrip uses a metallic base, while the wave antenna utilizes the natural earth with a finite conductivity. The conventional micro-strip antenna, however, is quite different in principle and operation from the wave antenna and the present one due to the different effects arising from high and low conductivity of the ground return. Therefore, this invention relates rather to the wave antenna in the sense that a traveling wave induced in the wire is further coupled to an incident sky wave.
FIG. 6 shows a schematic view of the wave antenna as described in an article entitled "The Wave Antenna--A New Type of Highly Directive Antenna" by H. H. Beverage, C. W. Rice, and E. W. Kellogg, which was published in a periodical, A.I.E.E., Vol. 42. Feb., 1923, pp. 215-266 [Ref. 1], where 9 is a conducting wire of the order of one wavelength long and several to ten meters above the earth and is installed within the plane of wave incidence, 10 is the earth return, 3 is the input end on the transmitter side, 4 is the receiver end, and 5 is a receiving set. When the signal wave reaches the antenna, an electromotive force is induced in the horizontal wire. A small current thus induced in each element of the wire starts to flow toward the receiver end, where the total current cumulated by successive additions is led to the receiver 5. Thus, the wave antenna utilizes the fact that the amplitude of the induced current becomes a maximum at a point on the wire 9.
The conventional wave antenna mentioned above has so far been used for long-wave communications in the range below 100 KHz in frequency or above 3 Km in wavelength, and the velocity of the induced wave on the wire, i.e., a line wave, is somewhat less than the velocity of light (slow wave), while the front velocity of the incident sky wave along the wire is higher than the velocity of light (fast wave) for an oblique incidence, and is equal to the velocity of light for horizontal incidence. Due to this difference in velocity between the line and sky waves, interference effects develop, the induced current on the wire building up at first for a certain distance and then decreasing in amplitude, as illustrated by the dashed line b in FIG. 3. (see also FIG. 2 at page 216 in Ref.1) This is because a phase difference between the incident sky wave and the induced line wave along the wire 9 caused by the difference in velocities acts to reduce the line current. In other words, the induced line wave couples only weakly to the sky wave for horizontal or near horizontal incidence. Thus, the amplification of the induced line current element itself does not occur for the conventional wave antenna, its attenuation constant still being equal to that of the eigen-wave of the line, .alpha..sub.0 (&gt;0) (damping wave). Consequently, the gain for the conventional wave antenna remains rather small.